Merge pull request #240 from iyoungm/master

backtracking 8queens

php/39_backtracking/queens.php

@@ -0,0 +1,74 @@
+<?php
+
+/**
+ * 8皇后解法，共92种解法 回溯思想
+ * Class Queen
+ */
+class Queen
+{
+    public $result = [];
+
+    function cal8queens($row)
+    {
+        if ($row == 8) {
+            $this->printQueens();
+            return;
+        }
+
+        //每一行有8中放法
+        for($column = 0; $column < 8; $column++) {
+            if ($this->isOk($row, $column)) {
+                $this->result[$row] = $column;
+                $this->cal8queens($row + 1);
+            }
+        }
+    }
+
+    //row行的column列是否合适
+    function isOk($row, $column)
+    {
+        $leftup = $column - 1;
+        $rightdown = $column + 1;
+
+        for ($i = $row - 1; $i >= 0; $i--) {
+            //判断上一行的 column 列是否有值
+            if ($this->result[$i] == $column) {
+                return false;
+            }
+
+            //左上角是否有值
+            if ($leftup >= 0 && $this->result[$i] == $leftup) {
+                return false;
+            }
+
+            //右下角是否有值
+            if ($rightdown < 8 && $this->result[$i] == $rightdown) {
+                return false;
+            }
+
+            $leftup--;
+            $rightdown++;
+
+        }
+
+        return true;
+    }
+
+    //打印
+    function printQueens()
+    {
+        for ($row = 0; $row < 8; $row++) {
+            for ($column = 0; $column < 8; $column++) {
+                if ($this->result[$row] == $column) {
+                    echo 'Q';
+                } else {
+                    echo '*';
+                }
+            }
+            echo '<br>';
+        }
+    }
+}
+
+$queen = new Queen();
+$queen->cal8queens(0);